The present invention generally relates to automatic irrigation systems, and particularly to a moisture sensing device which will allow irrigation to occur only when the soil requires it.
Most automatic irrigation systems today use the same method of operation, namely time control. These irrigation systems employ a controller which acts as a multi-station timer which will turn the water sprinklers on and off at predetermined intervals. These predetermined turn on and duration times will be programmed into the controller by the homeowner, landscaper or turf manager.
One of the drawbacks of these time based irrigation systems is that the controller must be reprogrammed regularly to keep from over or under-watering the area to be irrigated at different times of the year. In order to dispense water to the field being irrigated most efficiently, the ideal irrigation system would water only when soil conditions warranted. Specifically, only the amount of water required to replace the amount of moisture expended through evapotranspiration ("ET") should be dispensed by the irrigation system. ET is the total amount of water which moves from the soil to the air in a given amount of time due to evaporation directly from the soil and the transpiring from the grass itself. The ET level rises with increases in sunshine, wind and temperature, and decreases in relative humidity.
Present methods for replacing the moisture lost to ET include lengthy calculations based on data collected every day relating to micro-climate conditions. While this method works well for watering a large agricultural complex, the cost is impractical for most turf grass applications. Another technique for controlling the amount of irrigation in response to changing climate conditions is the use of a device to sense the amount of moisture present in the soil. A number of moisture sensors have been proposed, as will be briefly discussed below.
The most accurate type of moisture sensor is generally considered to be the neutron probe. Neutron probes are used by soil engineers to do soil moisture sampling on a weekly basis at large farms. These devices are relatively costly and they also require the use of radioactive materials in their operation.
One very old method of sampling soil moisture levels is to use a type of material which will expand and contract with varying degrees of moisture, such as wood or sponge. The expansion and contraction will usually raise or lower a diaphragm which controls water flow either directly or through electrodes making contact with each other. One of the drawbacks to this technique of sensing soil moisture is that a generally higher degree of accuracy is required for regulating an irrigation controller.
Another type of moisture sensor uses heat diffusion to determine if enough moisture is present in the soil to prevent watering. An example of such a moisture sensor is set forth in the U.S. Pat. No. 3,847,351, issued to Hasenbeck on Nov. 12, 1974. Generally speaking, this type of moisture sensor employs a thermistor which is located in a number of glass beads which are in turn surrounded by a porous material which allows water to come and go. When the system activates, the thermistor will heat up and the electronics for the sensor will measure the heat diffusion. If the soil is wet, and hence the beads are wet, then the heat will diffuse rapidly and the sensor electronics will prevent a solenoid from turning on and watering the field to be irrigated. If the beads are dry, then the heat will not diffuse as rapidly and the sensor electronics will permit the solenoid to be energized.
Another type of moisture sensor uses vacuum as a measure of the moisture in the soil. For example, tensiometers, which measure soil vacuum pressure in centibars, are the most widely accepted sensor for regulating the operation of a irrigation controller. The tensiometers are planted in wet soil and their gauges are initially set to zero. As the turf expends moisture through ET, additional moisture is drawn up from the soil through capillary action. This produces a vacuum which increases as the amount of moisture decreases. When the tensiometer gauge reaches a preset vacuum level, magnetic pick-ups complete a circuit which will allow the controller to begin irrigating the field.
Another type of moisture sensor is based on an electrical resistance which varies with the moisture level. Typically, in this type of sensor, two metal probes are separated by a couple of inches in a gypsum block. A regulated voltage is applied to one of the probes, and the voltage returning from the other probe is compared to it. The amount of current passing between the probes is dependent upon the level of moisture in the soil, because water conducts electricity. Besides gypsum blocks, other materials have been used as the porous sensor medium, including various granular materials and the soil itself. However, one of the drawbacks of this sensor design is that a wider range of resistance variation is needed to provide a good correlation to the exact amount of moisture in the soil.
Another form of moisture sensor is described in U.S. Pat. No. 3,626,286, issued to Pauchwerger on Dec. 7, 1971. This patent describes a sensor in which two plates or electrodes are separated and placed in the soil itself. The plates hold more electrons when the soil is dry because fewer of the electrons can leak to hte other side due to the presence of conductive moisture in the soil between the plates. An electronic circuit is also described which measures the capacitance of the probes or plates. This capacitance value is used to determine if enough moisture exists in the soil to prevent the watering of the soil. One of the drawbacks to this type of moisture sensor is that the soil itself is used as the dielectric material of the capacitor. This causes a variation in the performance of the moisture sensor due to differing soil compositions.
Accordingly, it is a principal objective of the present invention to provide a moisture sensing device which will permit the efficient replacement of water lost to ET without requiring the collection of micro-climate data.
It is another objective of the present invention to provide a moisture sensing device which is capable of being retrofitted into existing automatic irrigation systems using standard time control operation.
It is another objective of the present invention to provide a moisture sensing device whose accuracy is not dependent upon the composition of the soil being tested.
It is also an objective of the present invention to provide a moisture sensing device which is capable of automatically acting as a delay mechanism when the moisture level in the soil is close to being dry enough to allow for full cycle watering.